This invention relates to electrical power transfer apparatus.
This invention has particular but not exclusive application to the transfer of electrical power from photovoltaic cells to electrical loads or energy storage devices, and for illustrative purposes reference will be made to such application. However, it is to be understood that this invention could be used in other electrical power generation applications in which is exhibited a predictable relationship between output power and a measurable operating characteristic, such as in thermoelectric converters.
Electrical power sources such as photovoltaic cells have a potential power output which varies widely according to the intensity of light to which they are exposed. In addition, for a given level of light intensity (insolation) such cells have a characteristic power output curve which relates the actual output power of a cell or array of cells to the resistance of the electrical load applied to them.
Photovoltaic cells individually have a relatively low power output, and are usually used in arrays of cells electrically connected in series, parallel, or series-parallel configurations. For the purposes of this specification, an array is to be taken to mean any number of cells connected in any useful configuration.
When the load resistance is infinite, the current is zero, and the power output, which is the product of voltage and current, is also zero. The voltage of a cell at this operating point is called the open circuit voltage. When the load resistance falls to zero, the short circuit current flows and the output voltage falls to zero. The power output at this point must consequently be zero as well.
For load resistances between infinity and zero, the product of the output voltage and the output current gives a measure of the output power of a photovoltaic cell. The voltage, current and power output as a function of the load resistance may be plotted for a particular cell. The plot is characteristically a smooth curve with a maximum output at a certain value of load resistance. A family of such curves may be plotted, each showing the cell characteristics at a certain value of insolation. For each curve, the power output peak occurs at a different value of load resistance.
In most applications, such as producing electric power from solar radiation, the insolation varies continually, as the sun rises and falls in the sky during the day, and as a result of changing cloud cover. The highest energy output over a period of time from a photovoltaic cell or an array of cells operating under conditions of varying insolation will be achieved if the resistance of the load applied to the cell is adjusted continually to ensure that the values of load resistance, voltage and current which lead to the highest output power at a particular value of insolation are always attained.
A further problem with photovoltaic cells is that the voltage at which they produce maximum power may bear no relationship to the voltage level required for certain applications such as battery charging or operating appliances where a fixed output voltage is necessary. This problem may be overcome by utilising a switching power converter. This is an electronic device which converts available input power to different voltage and current levels. Conventional switching power converters used with photovoltaic cells are unable to maximise the power delivery at any time, as they are unable to change their operating parameters to ensure that the photovoltaic cells operate at their maximum power point.